A stroke-enlarging type brake booster mentioned herein means, as already disclosed in JITSU-KAI-SHO-No. 55 (1980)-22279 (Japanese Utility Model Application laid open to public), one wherein a power piston, which is so disposed in a booster casing as to divide the same into two chambers for being actuated by the pressure difference between the two, is separated from a controlling mechanism for controlling the pressure difference due to an operation of an input member for being relatively movable to the controlling mechanism. This stroke-enlarging type brake booster is featured in obtaining a larger output stroke than the input stroke, while in ordinary brake boosters the output stroke can never be larger than the input stroke.
In a kind of stroke-enlarging type brake boosters the power piston advances independently in the initial stage of the braking operation, and it must be advanced fairly rapidly for getting a better response to the braking operation. So the power piston inevitably hits a control piston, a part of the controlling mechanism, with some speed at the end of the advancing stroke thereof either directly or indirectly via another member. The impact sound produced by the collision of the two pistons is likely to cause the driver anxiety that trouble is happening as well as causing a disagreeable brake feeling due to the shock coming to the brake pedal. Still another disadvantage is the unsmooth brake depressing feeling felt by the driver due to the sudden change of driving force applied to the power piston before and after the collision of the two pistons.